Current power tools generally control rotational speed of an electric motor by controlling a duty cycle control turn-on of the electric motor. Some current power tools such as electric lawnmowers obtain a constant rotational speed by outputting a constant duty cycle. During an actual operation process of the tools, e.g., during a mowing process of electric lawnmowers, since variation of density degree of grasses would result in variation of load of the electric motor, under a constant duty cycle, the rotational speed of the electric motor, according to characteristics of the electric motor itself, would be varied with the variation of the load, that is, the rotational speed is reduced when the load is increased and increased when the load is reduced. However, users usually desire a higher rotational speed in the case of dense grasses and a lower rotational speed in the case of sparse grasses.